As is well known in the art, significant advances have been made in telecommunications systems over recent years, particularly in the rate at which information can be communicated. For example, conventional digital telecommunications systems provide extremely high bandwidth, such as the 44.736 Mbps data rate provided by the DS-3 data frame standard. Fiber optic-based systems can provide even higher bandwidth and data rates by way of time-division multiplexing of up to twelve DS-3 lines, resulting in bandwidth of up to 536.8 Mbps.
These extremely high bandwidth now available in digital telecommunications systems have enabled the communication of large volumes of data at high speeds. As a result, digital data is now commonly communicated from computer-to-computer. However, while digital voice transmissions can tolerate a significant amount of errors before the message is audibly distorted, each bit in a digital data transmission has significance. As such, high reliability and high quality transmission is necessary for the successful transmission of digital computer data. From the standpoint of the telecommunications provider, higher tariffs may be charged for those customers who require high quality digital transmission, with the level of quality defined by a bit error ratio limit (i.e., number of errored bits over the number of total bits transmitted).
Conventional digital communications systems provide such high quality transmission by switching between redundant facility paths in response to "performance monitoring" (commonly referred to as "PM") of the transmitted signal. The bit error ratio of received digital data is monitored, for example by way of cyclic redundancy check (CRC) and other conventional coding techniques; upon the monitoring process detecting a failure condition, the system either indicates an alarm to an operator or automatically switches the transmission. Performance monitoring and protection switching thus allows the telecommunications provider to ensure the desired grade of service desired by those customers paying premium tariffs for high quality and low error rate communications.
Failure conditions that are used to effect protection switching include "hard" failures, such as loss-of-signal ("LOS"), loss-of-frame ("LOF"), excessive bit error ratio ("excessive BER"), or alarm indication signal ("AIS"). Conventional fiber-optic terminals (FOTs) with 1:1 redundancy for the fiber optic lines in a system use these hard failures to automatically switch its transmission to the other of the two fiber optic lines responsive to receiving such hard failure indications.
However, many conditions other than hard failure conditions are also unacceptable to the telecommunication customer that is demanding high quality communication, particularly where computer data is to be transmitted and received. These other conditions may not be of such degree as to cause total loss of communication, but instead may degrade the quality of communication to an unacceptable extent. These other conditions include "soft" failures in which a particular signal has experienced a high bit error ratio. U.S. Pat. No. 5,329,520, issued Jul. 12, 1994, assigned to Alcatel Network Systems, Inc. and incorporated herein by this reference, describes a system that detects the number of bit errors over time, and that effects protection switching responsive to the detected bit error ratio exceeding certain thresholds.
As is well known in the art, protection switching is required to complete within a certain short period of time, (e.g., within 60 msec from the start of most failure conditions, according to certain industry standards), in order for the facility to be considered of sufficiently high quality as to bear an increased tariff. The above-incorporated U.S. Pat. No. 5,329,520 describes certain techniques for providing such rapid switching in the event of both hard and soft failure conditions.
Certain transient physical conditions are known to produce a heavily errored condition for a brief period of time, following which the transmission is again free from errors. These errored conditions are commonly referred to as "error bursts", or simply as "bursts". It has been observed that many bursts have a duration that is shorter than the time required for effecting protection switching. For those instances where such a short burst occurs, therefore, conventional bit-error-ratio protection switching techniques will introduce more error into the transmission from the lost traffic during the switching time, than is present due to the burst. The quality of service provided is therefore degraded by the protection switching from that which the transmission would have provided without the switching, even considering the burst.
It is therefore an object of the present invention to provide an apparatus and method for detecting burst conditions in a digital communications system.
It is a further object of the present invention to provide such an apparatus and method that suppresses protection switching responsive to detection of a burst condition.
It is a further object of the present invention to provide such an apparatus and method that detects such burst conditions in a manner that is independent of the alignment of the sample period with the burst.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.